Diode laser

ABSTRACT

A semiconductor diode laser has been fabricated from a leadgermanium telluride cyrstal. For example, a Pb0.95Ge0.05Te diode has been found to exhibit peak emission at a wavelength of 4.45 Mu . Compositional tuning of the device enables the selection of any desired peak emission wavelength within the approximate range of 2 to 6 Mu , which is useful for the spectroscopic determination of atmospheric gases and/or pollutants, including particularly carbon monoxide, carbon dioxide and nitric oxide.

United States Patent [191, Wrobel et al. I

[ I DIODE LASER [451 Mar. 26, 1974 OTHER PUBLICATIONS Square WaveOperation [75] Inventors: Joseph S. Wrobel, Garland; Robert Wooney et a!some Properties of GeTeLPbTe AL Thomas Bate Rlchardson, both 9 loysJour. of the Electrochemical 800., Jan. 1965, pp. 82-84. [73] Assignee:Texas Instruments Incorporated,

Dallas, Primary ExaminerRonald L. W1bert Attorney, Agent, or FirmHaroldLevine; Andrew M. 1 Flled= July 1971 Hassell; William E. I-Iiller [21]Appl. No.: 163,819

[57] ABSTRACT 52] U S C] 331/94 5 317/235 A Asemiconductor diode laserhas been fabricated from [51] 'f Hols 3/00 a lead-germanium telluridecyrstal. For example, a [58] Field of g i /235 AP Pb Ge Te diode hasbeen found to exhibit peak emission at a wavelength of 4.45;/..Compositional tun- ['S6] References Cited ing of the device enables theselection of any desired peak emission wavelength within the approximateUNITED STATES PATENTS range of 2 to 6 1., which is useful for thespectroscopic 3,482,189 12/1969 Fenner 331/945 determination ofatmospheric gases and/or pollutants, gggg izg including particularlycarbon monoxide, carbon diox- 3:624:553 1 H1971 ide and nitric oxide.3,059,1 17 10/1962 Boyle et a1 331/94.5 6 Claims, 7 Drawing Figures 15?Cur rent Monitor t Square Wave Amplifier Generator 27. 505- Output 24HgCdTe 29 [2 Laser Detector 2 2 LASER OR 2/ 3/ BEAM Lock-1n Amp 1 SOURCETrigger XY Recorder 12 PAIENIEB I SHEET 1 [IF 2 I /Z PHOTOMASK AND ETCHI CLEAVE INTO STRIPS o T 3 WE m m E L c c MOUNT ON TO-IB PATENTEBIARZSI97 'SHEET 2 OF 2 Laser Line (2 Amp Pulses, 300 pps 600 MA Threshold)1Y2 Current Monitor Square Wave F/g, 7 Amphfier Generator 509 OutputHgCdTe 29 Z 242 Detector Laser 22 LASER 28 a BEAM L0ck-1n Amp SOURCE JTrigger X-Y Recorder @2 Square Wave Operation M moms LASER Thisinvention relates to the fabrication of semiconductor lasers, and moreparticularly to leadgermanium telluride as a semiconductor material foruse in the fabrication of laser devices.

Infrared absorption spectroscopy is a classical vmethod for thedetection and quantitative determina tion of numerous gases and vapors.However, the instruments currently in use have resolutions that areinadequate for the analysis of certain typical gases, due

to the narrow absorption linewidths of such gases, and the inherentlimitations of the incoherent light sources used in standardinstruments. The use of gas lasers as coherent light sources is seldomsatisfactory, since these lasers cannot be tuned appreciably from theirnominal wavelengths. v

The use of certain semiconductor diode lasers in the design of infraredspectrometers has now been recognized as advantageous because they aretunable over a wide range of wavelengths, and because of their relativesimplicity, efficiency, and small size.

Accordingly, it isan object of thepresent invention to provide a tunablesemiconductor laser having superior operating characteristics,particularly for use in the 2 to ouregion of the infrared spectrum.-

It is a further object of the invention to provide a tunablesemiconductor diode laser having a wide range of peak emissionwavelengths, and the capability of operation at'higher temperatures thanprior semiconductor laser's of'comparable emission wavelengths. Stillfurther it is an object of the invention to provide a superior systemfor spectroscopic analysis of gases and vapors having infraredabsorption lines in the 2 to 6 p. region.

One aspect of the invention is embodied in a semiconductor lasercomprising lead-germanium telluride. In a preferred embodiment, thelaser crystal includes a p-n junction which can be stimulated to emitcoher ently light by forward biasing. However, in other embodiments thecrystal consists essentially of one conductivity' type, and isstimulated or pumped" by an electron beam, for example, or by a laserbeam emitted from another device.

A semiconductor diode laser, as distinguished from semiconductor diodesgenerally, or as distinguished from incoherent light emittingsemiconductor diodes, is shaped in the form of a Fabry-Perot opticalcavity. That is, the semiconductor body is preferably shaped to provideparallel, optically flat surfaces on opposite sides of the p-n junctionand perpendicular to at least a'substantial portion of the junction.Such geometry is significant because the parallel opposite surfacesoperate to reflect a sufficient portion of the spontaneous emission,within the plane of the junction, thereby stimulating the emission ofcoherent light. Other geometries are also capable of generating similarreflection patterns to enhance the lasing effect, including aprismshaped cavity, and a cylindrical cavity, for example. The geometryof the crystal is therefore not a critical feature of the broadestaspects of the invention, notwithstanding the fact that for mostcommercial embodiments a careful attention to geometry is required.

Another aspect of the invention is embodied in an analytical systemcomprising a lead-germanium telluride laser in combination with aninfrared detector in the optical path of the laser, and furtherincluding a gas cell or other sample holder between the laser anddetector, for spectroscopic detection or quantitative determination ofthe gases and/or'vapors therein having infrared absorption bands thatmatch the wavelength of the laser output. The analysis of solids is alsopossible.

FIGS. 1, 2, 3 and '4 are greatly enlarged perspective views of alead-germanium telluride wafer, or portions thereof, during variousstages of a preferred process for the fabrication of one embodiment ofthe laser of the invention;

FIG. 5 is a cross-sectional view of a lead-germanium telluride laserdevice of the invention;

FIG. 6 is a spectrogram of the output of one embodiment of the laser ofthe invention;

. FIG. 7 is a schematic diagramof an embodiment of the spectroscopicsystem of the invention.

As shown in FIG. 1, the illustrated process of fabrication begins with amonocrystalline wafer 11 of leadgermanium telluride having p-typeconductivity and crystallographically oriented in the l00 direction. Thewafer is obtained in accordance with any of the techniques previouslyknown for the growth of mixed monocrystals, such as in the growth oflead-tin telluride for example. Such techniques usually involve theheteroepitaxial growth of the mixed crystal on a substrate seed ofmonocrystalline lead telluride by vapor phase transport within a sealedquartz ampoule.

A p-n junction 12 is then formed within the crystal by the uniformdiffusion of antimony into one surface thereof to form a thin surfacelayer of n-typeconductivity. Suitable conditions for antimony diffusionhave been found to include a preliminary flash evaporation of elementalantimony on the crystal surface, followed by heating in a vacuum at atemperature at about 700C. for 3 to 4 hours.

7 As shown in FIG. 2, the structure of FIG. 1 is then provided with anetch resistant mask, pattern from a film of KTFR, for example, inaccordance with known photolithographic techniques. The wafer is etchedwith an aqueous solution of HBr and bromine to provide a plurality ofparallel strip mesas. Then, as shown in FIG. 3, the wafer is separatedby cleaving along the l00 direction to provide individual strips fromwhich futher cleaving perpendicular to the first cleaving, yields theindividual chips shaped in the form of an optical cavity suitable forachieving laser action, as illustrated in FIGQ4. The basic technologyrequired for selective etching and for cleaving is the same aspreviously known in the handling of related crystals, such as leadtelluride. As shown in FIG. 5, each individual chip is then mounted on aTO-l8 header and provided with ohmic connections in accordance withknown techniques, e.g., a Au-Tl alloy (p-type) and indium (ntype). j

The device of FIG. 5 was then mounted onto the cold finger of acryogenic dewar flask and cooled to liquid helium temperatures. Thedewar was fitted with optical windows and the laser output was focusedon the entrance slit of a monochromator. A cooled mercurycadmiumtelluride detector was focused on the exit slit of the monochromator.The diode was forward biased by one-microsecond current pulses at apulse rate of approximately 300 per second. At a threshold of about 600mA laser emission from the diode was observed. A spectrum of the emittedradiation was obtained using a 300 micron slit on the monochromator, andrecorded as shown in FIG. 6. The spectrum of FIG. 6 is specific 22. Agas cell or other sample holder (not shown) is located between the laserand the detector. The detector output is fed to amplifier 24, and theamplifier output is then passed to recorder 25.

The laser is operated by a pulse received from current monitor 27 whichin turn is supplied with a suitable pulse from square wave generator 28and amplifier 29. A trigger pulse is received by the square wavegenerator from amplifier 24.

A suitable mercury-cadmium telluride infrared detector system iscommercially available. Similarly, a system of equipment for pulsing thelaser is also commercially available as will be apparent to one skilledin the art. For example, a Tektronics 114 squarewave generator issuitable, in combination with Hewlett- Packard 467A amplifier.Alternatively, the laser 22 may be excited by another laser 31 or othersources of an electron beam.

The system of the invention is useful in the analytical detection andquantitative. determination of a wide range of gases and vapors. It isparticularly significant that compositional tuning of the laser of theinvention permits the selection of any desired peak emission wavelengthwithin the approximate range of 26p., which is particularly suitable inthe monitoring of atmospheric gases for nitric oxide, carbon monoxideand carbon dioxide. Other applications include the monitoring of theoxides of nitrogen, and for process control in chemical manufacturingplants.

Although the illustrated embodiment of the laser of the inventionemploys a semiconductor crysal Pb Ge 'l'e where x equals 0.05 x valuesfrom 0.001 to 0.25 are also within the scope of the invention. Tuning ofthe output wavelength is achieved by altering the ratio of germanium tolead, and is also achieved by altering the ratio of germanium to lead,and is also achieved to a lesser degree by varying the laser current ortemperature, by applying an external magnetic field, and by applyingpressure.

What is claimed is:

1. A semiconductor laser comprising a leadgermanium telluride mixedcrystal, said crystal including a p-n junction in combination withexcitation means for exciting said crystal above the threshold ofcoherent light emission, said excitation means comprising a source ofelectric current for biasing said junction above the threshold currentdensity for coherent light emission.

2. A semiconductor laser comprising a lead germanium telluride mixedcrystal, in combination with means for exciting said crystal above thethreshold of coherent light emission, said excitation means for excitingsaid crystal including means for generating an electron beam. g

3. A semiconductor laser comprising a leadgermanium telluridemixedcrystal, in combnationwith means for exciting said crystal above thethreshold of coherent light emission, said excitation means for excitingsaid crystal including a separate laser device for pumping said crystal.

4. A semiconductor diode laser comprising a mixed crystalmono-crystalline body of lead-germanium telluride having a pm junctiontherein, said body including opposite, parallel, optically flat surfacesperpendicular to a substantial portion of said junction.

5. A laser as defined by claim 4, wherein said body includes a p-typesubstrate, a shallow n-type surface layer, and an ohmic contact to eachof said substrate and said surface layer.

6. A laser as defined by claim 5, further including means for excitingsaid junction to cause the stimulated emission of coherent light.

2. A semiconductor laser comprising a lead germanium telluride mixedcrystal, in combination with means for exciting said crystal above thethreshold of coherent light emission, said excitation means for excitingsaid crystal including means for generating an electron beam. 3.Asemiconductor laser comprisinga lead-germanium telluridemixed crystal,in combnationwith means for exciting said crystal above the threshold ofcoherent light emission, said excitation means for exciting said crystalincluding a separate laser device for pumping said crystal.
 4. Asemiconductor diode laser comprising a mixed crystal mono-crystallinebody of lead-germanium telluride having a p-n junction therein, saidbody including opposite, parallel, optically flat surfaces perpendicularto a substantial portion of said junction.
 5. A laser as defined byclaim 4, wherein said body includes a p-type substrate, a shallow n-typesurface layer, and an ohmic contact to each of said substrate and saidsurface layer.
 6. A laser as defined by claim 5, further including meansfor exciting said junction to cause the stimulated emission of coherentlight.